Manufacture of nuclear fuel compacts

ABSTRACT

Nuclear fuel rods are manufactured utilizing a graphite flour-pitch matrix formulation containing an additive. The matrix formulation has a decreased viscosity at fabrication temperatures which permits manufacture of the fuel rods with lower fabrication pressures. Also, the matrix formulation does not cause the fuel rod to adhere or bond to the fuel element during heat treatment of the fuel rod in the fuel element. The nuclear fuel rods are suitable for use in high temperature gas cooled nuclear reactors.

This is a division, of application Ser. No. 510,390, filed Sept. 30,1974, now U.S. Pat. No. 4,064,204.

The present invention relates generally to improvements in themanufacture of nuclear fuel materials suitable for use in hightemperature gas cooled nuclear reactors. More particularly, the presentinvention relates to a composition suitable for the manufacture ofnuclear fuel compacts at lower fabrication pressures and without the useof externally applied mold release agents.

Nuclear fuel compacts in the form of rods are used in high temperaturegas cooled nuclear reactors. Such nuclear fuel rods are generallymanufactured by surrounding nuclear fuel particles with a matrix whichconsists of a mixture of graphite flour and pitch. As used herein, theterm "pitch" refers to residual products resulting from the destructivedistillation of such organic material as coal, petroleum and wood. Thenuclear fuel rods are formed in a metallic mold by either of twomethods. In one method, the mold is filled with nuclear fuel particlesand a hot graphite-pitch matrix is injected into the mold so as tosurround the nuclear fuel particles. In another method, a graphite-pitchmatrix is perpared and granulated. The granulated matrix is mixed withnuclear fuel particles to provide a molding mixture containing nuclearfuel particles. The molding mixture of granulated matrix and nuclearfuel particles is then filled into the mold. Thereafter the moldingmixture is heated and compressed within the mold. In both methods, afterthe fuel rod is formed in the mold, the mold is cooled and the fuel rodis ejected from the mold by pushing the fuel rod from the mold.

The viscosity of the matrix of graphite flour and pitch at thetemperature at which the fuel rod is formed by the above describedmethods is an important factor in determining the pressure required toprovide a suitably dense nuclear fuel compact. It is desirable to reducethe viscosity to permit lower fabrication pressures to be used.Excessive fabrication pressures can result in damage to the nuclear fuelparticles during formation of nuclear fuel rod. It would be desirable toprovide an additive for nuclear fuel molding mixtures which would act toreduce the fabrication pressure when fuel rods are formed from themolding mixture.

After ejection from the mold, the green fuel rod is placed in a graphitefuel element and is heated to between 1200 and 2000° C. to carbonize thehydrocarbons. The graphite fuel element usually has a hexagonally shapedcross section and an array of between about 50 to about 210 holes forreceiving the fuel rods. An approximately equal number of coolant holesextend through the length of the graphite fuel element. The heattreatment causes the binder material in the fuel rod matrix to undergosimultaneous decomposition and carbonization.

A plurality of green fuel rods are placed in each fuel hole. During thecarbonizing heat treatment the green fuel rods tend to slump and adhereto the walls of the fuel holes in the graphite fuel element. Suchadherence is detrimental to subsequent operation of the fuel element inthat differential expansion or contraction of the graphite fuel elementand the fuel rods may result in cracking of the fuel rods. It would bedesirable to reduce or eliminate the tendency of the fuel rods to adhereto the walls of the fuel holes during the carbonizing heat treatment.

Accordingly, it is a principal object of the present invention toprovide an improved method for the manufacture of nuclear fuel rodssuitable for use in high temperature gas cooled reactors. It is anotherobject of the present invention to provide a matrix formulation suitablefor use in the preparation of nuclear fuel rods which has a lowerviscosity at fabrication temperatures, thereby permitting fuel rodfabrication at lower temperatures and increased productivity. It is afurther object of the present invention to provide a matrix formulationsuitable for use in the preparation of nuclear fuel rods which decreasesor eliminates the adhesion of fuel rods to graphite fuel elements afterheating of the fuel rods in the graphite fuel elements.

Generally, in accordance with various features of the present invention,nuclear fuel rods are manufactured, utilizing a graphite flour-pitchmatrix formulation having an additive homogeneously dispersed therein.The matrix formulation has a decreased viscosity at fabricationtemperatures which permits manufacture of the fuel rods with lowerfabrication pressures. Also, the matrix formulation does not cause thefuel rod to adhere or bond to the fuel element during heat treatment ofthe fuel rod in the fuel element. The nuclear fuel rods are suitable foruse in high temperature gas cooled nuclear reactors.

In the method of manufacture, a matrix of pitch, and graphite flour isformed. The matrix is preferably formed by heating the pitch to anelevated temperature of from about 100° C. to about 300° C. andthoroughly mixing the graphite flour with the pitch at the elevatedtemperature to uniformly blend the graphite flour and pitch. The mixtureis thereafter cooled. Upon cooling the mixture is ground to provide aparticulate matrix with a particle size suitable for nuclear fuel rodformation. The matrix may also be formed by grinding the pitch to asuitable particle size and blending the graphite flour with the groundpitch to provide the matrix. The additive may be dispersed in the matrixeither during or after its preparation, as will be explained more fullyhereinafter.

In one embodiment of the invention, the matrix is blended with asuitable amount of a particulate nuclear fuel material to form a moldingmixture. Thereafter, nuclear fuel rods are formed by placing the moldingmixture into a steel mold and compressing the molding mixture in themold.

In another embodiment of the invention, the matrix is heated andinjected into a steel mold filled with particulate nuclear fuelmaterial.

In both embodiments, after ejection of the fuel rods from the mold, thefuel rods are placed in a graphite fuel element and are heated to atemperature of between about 1200 to 2000° C.

In accordance with the present invention, an additive is combined withthe matrix of graphite flour and pitch. The additive reduces theviscosity of the matrix at the temperature used in the fabrication ofthe fuel rods and reduces the tendency of the fuel rods to adhere to thegraphite fuel element after being heated in the fuel element. Theadditive also reduces the shear stress required to release the fuel rodsfrom the metal mold in which the fuel rods are fabricated. In thisconnection, a shear stress of less than about 50 psig is sufficient torelease a fuel rod from the metal mold when the additive of theinvention is present at the indicated levels.

In general, the additive is selected from saturated and unsaturatedalcohols having a carbon chain length of from 12 to 20, saturated andunsaturated fatty acids having a carbon chain length of from 12 to 20,saturated and unsaturated primary amines derived from fatty acids havinga carbon chain length of from 12 to 26 and saturated hydrocarbonsderived from petroleum having a molecular weight in the range of fromabout 350 to about 140. The additive is combined with the matrix at alevel sufficient to provide from about 0.5 percent to about 30 percentby weight of additive based on the weight of the molding mixture.

The nuclear fuel may be any fissionable or breeder nuclear fuel materialusually associated with the manufacture of gas cooled nuclear reactorfuel elements. Suitable nuclear fuel materials are diluted or undilutedpyrolytic carbon coated ThC₂, ThO, UO₂, UC₂, (Th,U)O₂, or (Th,U)C₂mixtures. The nuclear fuel material is preferably substantiallyspherical in shape and preferably has a particle size in the range offrom about 3 to about 1.2 mm.

The graphite flour may be derived from any carbonaceous material andpreferably has a particle size of at least less than about 0.040 mm.Preferably the graphite flour has a particle size in the range of fromabout 0.0002 to about 0.040 mm.

The pitch used in the matrix of the present invention may be any of theresidual products resulting from the destructive distillation of coal,petroleum and wood. The pitch has a softening point of less than about300° F. and has a viscosity in the range of from about 100 to about 1000poises at a temperature of 275° C. as measured by an Instron capillaryrheometer at a shear rate of 100 sec⁻¹.

More particularly, the additive of the present invention is preferablyselected from the group consisting of 1-octadecanol, 1-hexadecanol,oleic acid, stearic acid, 1-octadecylamine, petrolatum, and mixturesthereof. As indicated, the additive is used at a level of from about 0.5to about 30 percent by weight of the matrix of graphite flour and pitchand is preferably used at a level of from about 5 to about 15 percent byweight of the matrix. The additives of the present invention provide amatrix which has a viscosity of less than about 1000 poise as determinedby means of a capillary viscometer at a temperature of 175° C. and awall shear rate of 100 sec⁻¹.

It is preferred to combine the additive with the graphite flour andpitch of the matrix during formation of the matrix. This insures uniformdispersion of the additive in the matrix to provide a blend of theadditive and matrix. In this connection, a particularly preferred methodfor preparing the additive/matrix blend is to heat the pitch to atemperature where it is fluid, i.e., between 100° C. and 300° C. to mixthe additive with the heated pitch. Thereafter the graphite flour isadded to the mixture and the mixture is cooled and ground to provide amatrix with the additive uniformly dispersed therein. However, theadditive may be combined with the graphite flour or with ground pitchprior to forming the matrix. A uniform dispersion of the additive in thegraphite flour, ground pitch, heated pitch or particulate matrix can beeffected by suitable low shear mixing apparatus, such as a sigma blademixer.

The matrix and additive blend may be used "as is" to surround nuclearfuel particles which have been preloaded into a mold by injecting theheated matrix into the mold. In this method for making fuel rods, thematrix is heated to a temperature of from about 100° C. to about 300° C.Generally, injection pressures of from about 500 psig to about 3000 psigare suitable for injecting the matrix containing the additive of theinvention.

The matrix and additive blend may also be combined with nuclear fuelparticles prior to fabrication to provide a molding mixture suitable forcompression in a mold to fabricate nuclear fuel rods. In this method formaking fuel rods, the matrix and nuclear fuel particle combination areheated to a temperature of from about 100° C. to about 300° C. and arethen compressed. A fabrication pressure of from about 500 psig to about3000 psig is generally sufficient to compress the combination andprovide a green fuel rod suitable for further heat treatment.

In general, the matrix and additive blend comprises from about 20 toabout 50 percent by weight of graphite flour, from about 75 to about 30percent by weight of pitch and from about 0.5 to about 30 percent byweight of additive. The finished nuclear fuel rod comprises from about55 percent to about 64 percent of nuclear fuel material by volume andfrom about 45 to about 36 percent of the matrix and additive blend byvolume. As used herein all percentages are by weight, unless otherwiseexpressed.

As indicated, the additives of the present invention provides a matrixwith a reduced viscosity at fabrication temperatures and also provides afuel rod with improved resistance to adhesion with a fuel element. Also,the additives of the invention have mold release properties. The use ofadditives in the matrix enables manufacture of nuclear fuel compactswithout the use of mold release materials applied to the surface of themolds. The multi-functionality of the additives of the present inventionprovides a matrix for use in the manufacture of nuclear fuel compactswith greatly enhanced properties.

The following examples further illustrate various features of thepresent invention but are intended to in no way limit the scope of theinvention which is defined in the appended claims.

EXAMPLE I

Six hundred gm batches of a graphite flour-pitch matrix and additiveblend were prepared. The batches contained 342 gm of pitch, 240 gm ofgraphite flour having a particle size of less than about 0.04 and 18 gmof an additive.

A control batch was prepared wherein the additive was replaced with anadditional 18 gm of pitch. For each batch the pitch was heated to atemperature of 200° C., the additive was blended with the heated pitchand the graphite flour was then added. The mixture was then blended forthirty minutes in a sigma blade mixer at a temperature of 200° C. and ata mixing rate of 100 rpm.

After the batch was cooled, the batch was ground to provide a matrix andadditive blend having a particle size in the range of 0.6 to 0.9 mm. Theviscosity of the batches was then measured at 175° C. and a capillaryviscometer. Each of the batches was non-Newtonian showing a decrease inapparent viscosity with increasing shear rate. At a wall shear rate of100 sec⁻¹ the apparent viscosities of the batches, with variousadditives were as set forth below in Table I.

About 7 grams of the matrix and additive blend was mixed with about 20grams of a nuclear fuel material consisting of coated ThC₂ particles andhaving a particle size in the range of 0.6 to 0.9 mm.

Each of the batches was then used to prepare nuclear fuel rods byplacing the 27 gram batch (20 grams fuel particles, 7 grams matrix) intoa steel mold and compressing the batch at a temperature of 190° C. and apressure of 1200 psig. After the nuclear fuel rod was formed the wallshear stress necessary to push each of four consecutive rods out of thesame mold cavity with no cleaning of the molds between rods wasmeasured. The average wall shear stress value obtained for each of theadditives is indicated below in Table II.

Fuel rods fabricated as described above using matrix without additivesand fuel rods fabricated using matrix with additives were heat treatedto 1800° C. in graphite sleeves about 12 inches long, about 0.625 inchesinside diameter, and about 0.975 inches outside diameter with both endsplugged with graphite. It was found that the fuel rods fabricated usingmatrix without additives adhered to the graphite sleeve, while thosefabricated using matrix with additive did not.

                  Table I                                                         ______________________________________                                        Additive              Viscosity, Poise                                        ______________________________________                                        None                  2350                                                    Petrolatum (avg. molecular weight                                                                   610                                                     approx. 500)                                                                  1-Octodecanol         600                                                     Paraffin wax (avg. molecular weight                                                                 770                                                     approx. 700)                                                                  Oleic acid            920                                                     Stearic acid          660                                                     1-Hexadecanol         800                                                     Stearic acid + paraffin (50:50)                                                                     660                                                     Stearic acid + 1-Octadecanol (50:50)                                                                660                                                     1-Octadecylamine      800                                                     ______________________________________                                    

                  Table II                                                        ______________________________________                                        Additive              Shear Stress, PSI                                       ______________________________________                                        None                  1900                                                    Petrolatum            8.71                                                    1-Octadecanol         0.61                                                    Paraffin wax (avg. molecular weight                                                                 31.28                                                   approx. 700)                                                                  Oleic acid            2.11                                                    Stearic acid          0.73                                                    1-Hexadecanol         3.56                                                    Stearic acid + paraffin (50:50)                                                                     1.21                                                    Stearic acid + 1-Octadecanol (50:50)                                                                0.95                                                    ______________________________________                                    

EXAMPLE II

Further batches of a graphite flour, pitch and additive matrix wereprepared having the formulation set forth below in Table III. Theapparent viscosity of each of the batches was determined. Each of thematrix batches was then used to prepare fuel rods by the injectionmethod. In this method, about 20 grams of ThC₂ nuclear fuel particleshaving a particle size in the range of 0.6 to 0.9 mm, were placed in a15.9 mm diameter cylindrical steel mold. About 7 grams of coarselyground matrix having a particle size of about 0.6 mm were placed on topof the fuel particles in the mold. The mold was then heated to atemperature of 200° C. and a piston was used to force the matrix throughthe fuel particles to form fuel rods having a length of 61 mm. Theaverage shear stress required to remove four fuel rods from the mold wasmeasured and is reported below in Table III.

                  Table III                                                       ______________________________________                                                                        Apparent Av-                                  Graph-                          Viscosity                                                                              erage                                ite           Addi-             at 175° C.                                                                      Shear                                Flour-                                                                              Pitch.sup.1                                                                           tive    Additional                                                                              and 100  Stress-                              grams grams   grams   Type      sec.sup.-1 -poise                                                                      psig                                 ______________________________________                                        180.sup.2                                                                           402     18      1-Octadecanol                                                                           410      12.6                                 180.sup.2                                                                           384     36      "         420      14.5                                 198.sup.2                                                                           384     18      "         460      14.5                                 198.sup.2                                                                           384     60      "         270      10.6                                 240.sup.3                                                                           324     36      "         380      14.8                                 240.sup.4                                                                           324     36      "         260       2.0                                 ______________________________________                                         .sup.1 Obtained from Ashland Oil Co. and identified as A240                   .sup.2 Obtained from Asbury Graphite Corp. and identified as grade 6353       .sup.3 Obtained from Great Lakes Caron Co. and identified as grade 1089       .sup.4 Obtained from Lonza Ltd. and identified as grade KS44.            

By the present invention, a matrix composition with reduced viscosity isprovided for the manufacture of nuclear fuel rods intended for use ingas cooled nuclear reactors. The invention also provides a matrixcomposition that reduces the adhesion of the fuel rods to graphite fuelelements after heating the fuel rods in the fuel element.

What is claimed is:
 1. A matrix suitable for preparing nuclear fuel rodsbelow 300° said matrix comprising graphite flour, pitch and an additive,said additive being selected from saturated and unsaturated alcoholshaving a carbon chain length of from 12 to 20, saturated and unsaturatedfatty acids having a carbon chain length of from 12 to 20, saturated andunsaturated primary amines derived from fatty acids having a carbonchain length of from 12 to 20 and saturated hydrocarbons derived frompetroleum having a molecular weight in the range of from about 350 toabout 1400 said additive being present at a level of from about 2 toabout 12 percent by weight of said matrix, said graphite flour beingpresent at a level of from about 25 to about 45 percent, by weight ofsaid matrix, and said pitch being present at a level of from about 43 to73 percent by weight of said matrix.
 2. A matrix in accordance withclaim 1 wherein said additive is selected from the group consisting of1-octadecanol, 1-hexadecanol, oleic acid, stearic acid,1-octadecylamine, petrolatum, and mixtures thereof.
 3. A matrix inaccordance with claim 1 wherein said additive is present in said matrixat a level of from about 5 to about 10 percent by weight.
 4. A matrix inaccordance with claim 1 wherein said matrix has a viscosity of less thanabout 1000 poises as determined by a capillary viscometer at atemperature of 175° C. and a wall shear rate of 100 sec⁻¹.